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5 Functions
5.1 Information about a function
5.1-1 NameFunction
5.1-2 NumberArgumentsFunction
5.1-3 NamesLocalVariablesFunction
5.1-4 FilenameFunc
5.1-5 StartlineFunc
5.1-6 PageSource
5.2 Calling a function with a list argument that is interpreted as several arguments
5.2-1 CallFuncList
5.3 Functions that do nothing
5.3-1 ReturnTrue
5.3-2 ReturnFalse
5.3-3 ReturnFail
5.3-4 IdFunc
5.4 Function Types
5.4-1 IsFunction
5.4-2 IsOperation
5.4-3 FunctionsFamily
5.5 Naming Conventions

### 5 Functions

The section 4.23 describes how to define a function. In this chapter we describe functions that give information about functions, and various utility functions used either when defining functions or calling functions.

#### 5.1 Information about a function

##### 5.1-1 NameFunction

`‣ NameFunction`( func ) | ( function )

returns the name of a function. For operations, this is the name used in their declaration. For functions, this is the variable name they were first assigned to. (For some internal functions, this might be a name different from the name that is documented.) If no such name exists, the string `"unknown"` is returned.

    gap> NameFunction(SylowSubgroup);
    "SylowSubgroup"
    gap> Blubberflutsch:=x->x;;
    gap> NameFunction(Blubberflutsch);
    "Blubberflutsch"
    gap> a:=Blubberflutsch;;
    gap> NameFunction(a);
    "Blubberflutsch"
    gap> NameFunction(x->x);
    "unknown"
    gap> NameFunction(NameFunction);
    "NameFunction"

##### 5.1-2 NumberArgumentsFunction

`‣ NumberArgumentsFunction`( func ) | ( function )

returns the number of arguments the function func accepts. For functions that use `arg` to take a variable number of arguments, as well as for operations, -1 is returned. For attributes, 1 is returned.

    gap> NumberArgumentsFunction(function(a,b,c,d,e,f,g,h,i,j,k)return 1;end);
    11
    gap> NumberArgumentsFunction(Size);
    1
    gap> NumberArgumentsFunction(IsCollsCollsElms);
    3
    gap> NumberArgumentsFunction(Sum);
    -1

##### 5.1-3 NamesLocalVariablesFunction

`‣ NamesLocalVariablesFunction`( func ) | ( function )

returns a mutable list of strings; the first entries are the names of the arguments of the function func, in the same order as they were entered in the definition of func, and the remaining ones are the local variables as given in the `local` statement in func. (The number of arguments can be computed with `NumberArgumentsFunction` (5.1-2).)

    gap> NamesLocalVariablesFunction(function( a, b ) local c; return 1; end);
    [ "a", "b", "c" ]
    gap> NamesLocalVariablesFunction(function( arg ) local a; return 1; end);
    [ "arg", "a" ]
    gap> NamesLocalVariablesFunction( Size );
    fail

##### 5.1-4 FilenameFunc

`‣ FilenameFunc`( func ) | ( function )

For a function func, `FilenameFunc` returns either `fail` or the absolute path of the file from which func has been read. The return value `fail` occurs if func is a compiled function or an operation. For functions that have been entered interactively, the string `"*stdin*"` is returned, see Section 9.5.

    gap> FilenameFunc( LEN_LIST );  # a kernel function
    fail
    gap> FilenameFunc( Size );      # an operation
    fail
    gap> FilenameFunc( x -> x^2 );  # an interactively entered function
    "*stdin*"
    gap> meth:= ApplicableMethod( Size, [ Group( () ) ] );;
    gap> FilenameFunc( meth );
    "... some path .../grpperm.gi"

##### 5.1-5 StartlineFunc

`‣ StartlineFunc`( func ) | ( function )

`‣ EndlineFunc`( func ) | ( function )

Let func be a function. If `FilenameFunc` (5.1-4) returns `fail` for func then also `StartlineFunc` returns `fail`. If `FilenameFunc` (5.1-4) returns a filename for func then `StartlineFunc` returns the line number in this file where the definition of func starts.

`EndlineFunc` behaves similarly and returns the line number in this file where the definition of func ends.

    gap> meth:= ApplicableMethod( Size, [ Group( () ) ] );;
    gap> FilenameFunc( meth );
    "... some path ... gap4r5/lib/grpperm.gi"
    gap> StartlineFunc( meth );
    487
    gap> EndlineFunc( meth );
    487

##### 5.1-6 PageSource

`‣ PageSource`( func ) | ( function )

This shows the file containing the source code of the function or method func in a pager (see `Pager` (2.4-1)). The display starts at a line shortly before the code of func.

This function works if `FilenameFunc(func)` returns the name of a proper file. In that case this filename and the position of the function definition are also printed. Otherwise the function indicates that the source is not available (for example this happens for functions which are implemented in the GAP C-kernel).

Usage examples:
`met := ApplicableMethod(\^, [(1,2),2743527]); PageSource(met);`
`PageSource(Combinations);`
`ct:=CharacterTable(Group((1,2,3)));`
`met := ApplicableMethod(Size,[ct]); PageSource(met);`

#### 5.2 Calling a function with a list argument that is interpreted as several arguments

##### 5.2-1 CallFuncList

`‣ CallFuncList`( func, args ) | ( operation )

returns the result, when calling function func with the arguments given in the list args, i.e. args is "unwrapped" so that args appears as several arguments to func.

    gap> CallFuncList(\+, [6, 7]);
    13
    gap> #is equivalent to:
    gap> \+(6, 7);
    13

A more useful application of `CallFuncList` is for a function `g` that is called in the body of a function `f` with (a sublist of) the arguments of `f`, where `f` has been defined with a single formal argument `arg` (see 4.23), as in the following code fragment.

    f := function ( arg )
           CallFuncList(g, arg);
           ...
         end;

In the body of `f` the several arguments passed to `f` become a list `arg`. If `g` were called instead via `g( arg )` then `g` would see a single list argument, so that `g` would, in general, have to "unwrap" the passed list. The following (not particularly useful) example demonstrates both described possibilities for the call to `g`.

    gap> PrintNumberFromDigits := function ( arg )
    >     CallFuncList( Print, arg );
    >     Print( "\n" );
    >    end;
    function( arg ) ... end
    gap> PrintNumberFromDigits( 1, 9, 7, 3, 2 );
    19732
    gap> PrintDigits := function ( arg )
    >     Print( arg );
    >     Print( "\n" );
    >    end;
    function( arg ) ... end
    gap> PrintDigits( 1, 9, 7, 3, 2 );
    [ 1, 9, 7, 3, 2 ]

#### 5.3 Functions that do nothing

The following functions return fixed results (or just their own argument). They can be useful in places when the syntax requires a function, but actually no functionality is required. So `ReturnTrue` (5.3-1) is often used as family predicate in `InstallMethod` (78.2-1).

##### 5.3-1 ReturnTrue

`‣ ReturnTrue`( ... ) | ( function )

This function takes any number of arguments, and always returns `true`.

    gap> f:=ReturnTrue;
    function( arg ) ... end
    gap> f();
    true
    gap> f(42);
    true

##### 5.3-2 ReturnFalse

`‣ ReturnFalse`( ... ) | ( function )

This function takes any number of arguments, and always returns `false`.

    gap> f:=ReturnFalse;
    function( arg ) ... end
    gap> f();
    false
    gap> f("any_string");
    false

##### 5.3-3 ReturnFail

`‣ ReturnFail`( ... ) | ( function )

This function takes any number of arguments, and always returns `fail`.

    gap> oops:=ReturnFail;
    function( arg ) ... end
    gap> oops();
    fail
    gap> oops(-42);
    fail

##### 5.3-4 IdFunc

`‣ IdFunc`( obj ) | ( function )

returns obj.

    gap> id:=IdFunc;
    function( object ) ... end
    gap> id(42);
    42
    gap> f:=id(SymmetricGroup(3));
    Sym( [ 1 .. 3 ] )
    gap> s:=One(AutomorphismGroup(SymmetricGroup(3)));
    IdentityMapping( Sym( [ 1 .. 3 ] ) )
    gap> f=s;
    false

#### 5.4 Function Types

Functions are GAP objects and thus have categories and a family.

##### 5.4-1 IsFunction

`‣ IsFunction`( obj ) | ( category )

is the category of functions.

    gap> IsFunction(x->x^2);
    true
    gap> IsFunction(Factorial);
    true
    gap> f:=One(AutomorphismGroup(SymmetricGroup(3)));
    IdentityMapping( Sym( [ 1 .. 3 ] ) )
    gap> IsFunction(f);
    false

##### 5.4-2 IsOperation

`‣ IsOperation`( obj ) | ( category )

is the category of operations. Every operation is a function, but not vice versa.

    gap> MinimalPolynomial;
    <Operation "MinimalPolynomial">
    gap> IsOperation(MinimalPolynomial);
    true
    gap> IsFunction(MinimalPolynomial);
    true
    gap> Factorial;
    function( n ) ... end
    gap> IsOperation(Factorial);
    false

##### 5.4-3 FunctionsFamily

`‣ FunctionsFamily` | ( global variable )

is the family of all functions.

#### 5.5 Naming Conventions

The way functions are named in GAP might help to memorize or even guess names of library functions.

If a variable name consists of several words then the first letter of each word is capitalized.

If the first part of the name of a function is a verb then the function may modify its argument(s) but does not return anything, for example `Append` (21.4-5) appends the list given as second argument to the list given as first argument. Otherwise the function returns an object without changing the arguments, for example `Concatenation` (21.20-1) returns the concatenation of the lists given as arguments.

If the name of a function contains the word "`Of`" then the return value is thought of as information deduced from the arguments. Usually such functions are attributes (see 13.5). Examples are `GeneratorsOfGroup` (39.2-4), which returns a list of generators for the group entered as argument, or `DiagonalOfMat` (24.12-1).

For the setter and tester functions of an attribute `Attr` the names `SetAttr` resp. `HasAttr` are available (see 13.5).

If the name of a function contains the word "`By`" then the return value is thought of as built in a certain way from the parts given as arguments. For example, creating a group as a factor group of a given group by a normal subgroup can be done by taking the image of `NaturalHomomorphismByNormalSubgroup` (39.18-1). Other examples of "`By`" functions are `GroupHomomorphismByImages` (40.1-1) and `LaurentPolynomialByCoefficients` (66.13-1).

Often such functions construct an algebraic structure given by its generators (for example, `RingByGenerators` (56.1-4)). In some cases, "`By`" may be replaced by "`With`" (like e.g. `GroupWithGenerators` (39.2-3)) or even both versions of the name may be used. The difference between `StructByGenerators` and `StructWithGenerators` is that the latter guarantees that the `GeneratorsOfStruct` value of the result is equal to the given set of generators (see 31.3).

If the name of a function has the form "`AsSomething`" then the return value is an object (usually a collection which has the same family of elements), which may, for example:

- know more about its own structure (and so support more operations) than its input (e.g. if the elements of the collection form a group, then this group can be constructed using `AsGroup` (39.2-5));

- discard its additional structure (e.g. `AsList` (30.3-8) applied to a group will return a list of its elements);

- contain all elements of the original object without duplicates (like e.g. `AsSet` (30.3-10) does if its argument is a list of elements from the same family);

- remain unchanged (like e.g. `AsSemigroup` (51.1-5) does if its argument is a group).

If `Something` and the argument of `AsSomething` are domains, some further rules apply as explained in Tutorial: Changing the Structure.

If the name of a function `fun1` ends with "`NC`" then there is another function `fun2` with the same name except that the `NC` is missing. `NC` stands for "no check". When `fun2` is called then it checks whether its arguments are valid, and if so then it calls `fun1`. The functions `SubgroupNC` (39.3-1) and `Subgroup` (39.3-1) are a typical example.

The idea is that the possibly time consuming check of the arguments can be omitted if one is sure that they are unnecessary. For example, if an algorithm produces generators of the derived subgroup of a group then it is guaranteed that they lie in the original group; `Subgroup` (39.3-1) would check this, and `SubgroupNC` (39.3-1) omits the check.

Needless to say, all these rules are not followed slavishly, for example there is one operation `Zero` (31.10-3) instead of two operations `ZeroOfElement` and `ZeroOfAdditiveGroup`.

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